Molding of plastic parts is a process by which a plastic composition is formed into the geometry of the final part. Typically, the plastic is first heated beyond its melting point into its molten form. The molten plastic is then injected under high pressure into a mold cavity that has the geometric features of the final part. In order for the molten plastic to solidify, the mold may be cooled using a fluid such as water. The walls of the mold conduct heat from the molten plastic to water channels that carry the heat out of the tool.
Known molding processes are limited by the need for mold walls to be cold. The temperature of the mold is usually maintained at a level suggested by the manufacturer for the plastic. A colder mold provides faster solidification and quicker cycle times, resulting in lower cost parts. Warmer molds, however, allow the material to flow farther with less pressure, making the manufacturing process easier. In addition, the surface appearance of the part may be improved if the mold temperature is raised. Plastic compositions including with fillers, such as flake or fibrous fillers, are particularly difficult to mold into articles having a desirable surface appearance.
Some attempts have been made to overcome these problems using active heating of the mold surface. For example, U.S. Pat. No. 5,851,474 to Allan et al. generally describes an injection molding process in which inner surface areas of the mold are heated above the heat distortion temperature of the molded plastic. However, this method does not provide rapid molding cycles.
Other methods of mold temperature control are described in, for example, Japanese Patent Publication Nos. 09-314611, 09-314628, 10-100157, and 10-100216.
There is a need for an improved method and apparatus that provides rapid cycling between a higher temperature for filling the mold and a lower temperature for solidifying the article.